An example of a direct multiplexed, rms responding electronic display is the well-known liquid crystal display (LCD). In such displays, a nematic liquid crystal material is positioned between two parallel glass plates having electrodes applied to each surface in contact with the liquid crystal material. The electrodes typically are arranged in vertical columns on one plate and horizontal rows on the other plate for driving a picture element (pixel) wherever a column and row electrode overlap. A high information content display, e.g., a display used as a monitor in a portable laptop computer, requires a large number of pixels to portray arbitrary patterns of information. Matrix LCDs having four hundred eighty rows and six hundred forty columns forming 307,200 pixels are widely used in computers today, and matrix LCDs with millions of pixels are expected soon.
In so-called rms responding displays, the optical state of a pixel is substantially responsive to the square of the voltage applied to the pixel, i.e., the difference in the voltages applied to the electrodes on the opposite sides of the pixel. LCDs have an inherent time constant that characterizes the time required for the optical state of a pixel to return to an equilibrium state after the optical state has been modified by changing the voltage applied to the pixel. Recent technological advances have produced LCDs with time constants approaching the frame period used in many video displays (approximately 16.7 milliseconds). Such a short time constant allows the LCD to respond quickly and is especially advantageous for depicting motion without noticeable smearing of the displayed image.
An active addressing method is typically used to optimize the contrast ratio of an LCD being used for video information display. In the typically used active addressing method, video information consisting of frames of image values is organized in a sequence of rows of image values which are transmitted to the display system. Each image value represents a value (gray scale values in a black and white, gray scale system) of a pixel in the image which is to be presented at a pixel in the display. The active addressing method continuously drives the row electrodes with signals comprising a train of periodic pulses having a common period T corresponding to the frame period. The row signals are independent of the image to be displayed and preferably are orthogonal and normalized, i.e., orthonormal. The term orthogonal denotes that if the amplitude of a signal applied to one of the rows is multiplied by the amplitude of a signal applied to another one of the rows, the integral of this product over the frame period is zero. The term normalized denotes that all the row signals have the same rms voltage integrated over the frame period T.
A problem with active addressing results from the large number of calculations required per second. For example, a gray scale display having four hundred eighty rows and six hundred forty columns, and a frame rate of 60 frames per second requires just under ten billion calculations per second. Typical currently available display systems using active addressing have two sets of video image memory, each set capable of storing the four hundred eighty by six hundred forty image values, each image value being typically an eight bit value. One of the sets of memory is used to assemble a frame of image values on a row by row basis, while the second set of memory is used as a source of image values in which columns of the image values remain constant for a frame period. Such constancy of column information is important to prevent jitter and smearing of the image. Although it is possible with today's technology to perform calculations at the rate described, the architectures proposed to date for calculation engines used for actively addressed displays have not been optimized to minimize memory requirements. The memory requirement issue is particularly important in portable applications, wherein excessive memory results in an excessive power requirement, larger parts, and a higher cost of the memory. The excessive power requirement is particularly important in such portable applications as battery-powered laptop computers, in which size, and battery life are primary design considerations.
Thus, what is needed is a method and apparatus for controlling and driving an actively addressed display in a manner that minimizes the memory requirements and thus also minimizes the power consumption and size of the image processing system.